It is known for the operation of an engine to control ignition, fuel injection and other functions of the engine with an engine control unit. Typical input signals for such a controller are, among others, the number of revolutions of the engine per minute, the position of a pedal, e.g. an accelerator pedal as well as the temperature and the pressure of the fuel. With the objective of a precise regulation of the fuel injection in a common rail injection system it is desirable to know the fuel temperature at several places of the common rail injection system, especially to know the fuel temperature inside the common rail and at the position of the injectors.
It is expensive and technically difficult to measure the fuel temperature inside the common rail directly with a temperature sensor. In practice, the fuel temperature has only been determined at the leakage of the injectors. Injectors of the new generation however do not have enough leakage for measuring the temperature with sufficient accuracy. Therefore it is necessary to determine the fuel temperature inside the common rail and the leakage line of the injectors by other means.
The document DE 103 01 264 B4 discloses a method for determining the fuel temperature in a common rail injection system, whereby the pressure of the fuel in the common rail is measured with a pressure sensor, wherein the acoustic velocity of a pressure wave in the fuel is determined, that is created by the injection of fuel and detected by the pressure sensor. The temperature of the fuel is determined with the acoustic velocity of the pressure wave. Moreover the acoustic velocity is determined by the periodic time of the standing pressure wave. This method only allows to determine the fuel temperature in the proximity of the injectors.
The document DE 38 32 101 C2 discloses a method and an apparatus to measure the fuel temperature for electronically controlled engines with a direct injection system. The fuel is injected through injectors in which at least one comprises a needle lift sensor with a sensor coil. The temperature dependent DC resistance of the coil of the needle lift sensor is measured and is taken as a parameter for the fuel temperature. Also this method only allows to determine the fuel temperature at the injectors.